This invention relates to thin film heterojunction photovoltaic cells and methods of forming ohmic contacts with a thin film of a p-type semiconductor compound formed of at least one of the metal elements of Class II B of the Periodic Table of Elements and one of the non-metal elements of Class VI A of the Periodic Table of Elements.
The difficulty of making good ohmic contacts to p-type Class II B-VI A compounds such as cadmium telluride is well known. These materials on the other hand are very important for photovoltaic applications. As a consequence the utilization of such compounds for thin film solar cell applications largely depend on the ability to make low resistance ohmic contacts on them. Consequently, the following procedure has been widely accepted for use in making ohmic contacts to p-CdTe:
(a) etching the p-CdTe with an acidic solution and then (b) depositing by vacuum evaporation or electroless plating a suitable metal such as platinum, gold, nickel, or copper. The acidic etches commonly used in the first step are:
H.sub.2 SO.sub.4 +K.sub.2 Cr.sub.2 O.sub.7 +H.sub.2 O; HNO.sub.3 +K.sub.2 Cr.sub.2 O.sub.7 +H.sub.2 O; HNO.sub.3 +K.sub.2 Cr.sub.2 O.sub.7 +AgNO.sub.3 +H.sub.2 O; HF+H.sub.2 O.sub.2 +H.sub.2 O; HF+HNO.sub.3 +H.sub.2 O; HNO.sub.3 +H.sub.2 O and bromine methanol. The belief is that the acidic etch leaves a tellurium rich surface which improves the contact between the metal to be deposited and the p-CdTe. Such acidic etchings have been normally used in connection with low resistivity single crystal CdTe and contact deposition is sometimes followed by a 5-15 minute heat treatment (150.degree. C.) in H.sub.2 atmosphere to improve the contact. A recent study concludes that the best ohmic contact to single crystal low resistivity p-CdTe can be obtained by an etch (K.sub.2 Cr.sub.2 O.sub.7 +H.sub.2 SO.sub.4 +H.sub.2 O) followed by (Au+Cu) alloy evaporation.
In the case of thin film (thickness less than 10 micrometers) devices one works with a relatively high resistivity material (resistivity greater than 100 ohm-centimeters) which makes the contact problem ever more serious. Strong acidic etches used for single crystal CdTe are not normally used for thin films with the exception of very dilute (approximately 0.1%) bromine methanol. Among various techniques reported are direct application of the contact material on the fresh CdTe surface without any etching and formation of a Cu.sub.x Te layer on the surface by evaporation or by chemical methods. But by far the most common technique is contact deposition after a mild bromine methanol etch. Unfortunately, the use of the Cu.sub.x Te layer has a tendency to be unstable and the direct application of the contact metal as well as a simple bromine methanol etch does not give as good an ohmic contact as is generally desired.
Consequently, an object of the present invention is an improved method of forming ohmic contacts between a Class II B-VI A compound and a conductive metal layer. In particular, an object of the present invention is to form such ohmic contact with p-type cadmium telluride.